The conventional column oven approach in gas chromatography has many undesirable characteristics such as: bulk, high power requirements, cost, high thermal mass with low response times, and longer times between runs. The application of resistive heating to the metal cladding on capillary columns provide an improvement on column heating but introduce a temperature measurement challenge and inherent temperature measurement inaccuracy. There is a need for an accurate, responsive and programmable column temperature program.
Also, it is well known that column efficiency needed to generate sharp narrow chromatographic peaks is enhanced with a reduction in the internal diameter of the capillary tubing. Generally, a reduction in the internal diameter of the capillary tubing results in a reduction in the sample capacity, and requires specialized injection ports and more expensive sensitive detectors. There is a need for reproducibility in preparing multicapillary columns. There is a need for multicapillary columns that are feasible for a wide range of applications without the individual column chromatographic variability and injector detector interface problems that have arisen when multicapillary column applications have been attempted in the past.
Low thermal mass gas chromatograph (GC) columns are available but are often complex, having a combination of separate heating and sensor wires. Additionally, current low thermal mass GC columns are generally single tube columns lacking the sample capacity associated with high efficiency small internal diameter capillary columns.
There are also drawbacks with the current coating procedures for capillaries in GC column preparation. There are conventionally two stationary phase coating procedures for GC column preparation: dynamic and static coating procedures.
The dynamic coating procedure consists of a plug of coating solution, solvent containing the stationary phase, which is slowly moved through the tubing using gas pressure depositing stationary phase as the plug passes along the walls of the tubing. This method creates the most variable film thickness over the length of the tubing, which reduces the column efficiency.
The static coating procedure involves the loading of the tube with a coating solution consisting of the stationary phase and solvent usually chloroform or dichloromethane. Once the column is loaded the solvent is evaporated using low pressure at a constant temperature. Conventionally the pressure and temperature used to evaporate the solvent is about 100 mm Hg at approximately room temperature. However, the solvent front does not continuously move forward under these conditions. The solution moves toward the vacuum for a moment and then continues the evaporation process. This solution excursion causes a recoating of the walls of the tubing which creates variable film thickness. This variation in film thickness may not be apparent on single capillary columns but becomes very evident when comparing chromatographic data from multicapillary columns. The recoating process contributes to variable film thickness making the use of multicapillary columns impractical due to variations in retention factors and column efficiencies for each of the tubes within the multicapillary column.
If a coating solution is introduced to a capillary with helium gas pressure the dissolved gases may promote a flashing of the coating solution and leave the capillary devoid of the stationary phase. A high gas pressure may promote flashing due to gas being dissolved in the capillary. A conventional rinsing and coating reservoir using gas pressure to load the capillaries can result in an unacceptably high number of tubes that flash and be devoid of stationary phase.